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  • 143

    12

    Corrective and Preventive Maintenance

    12.1 INTRODUCTION

    Corrective maintenance is the remedial action performed because of failure ordeficiencies found during preventive maintenance or otherwise, to repair an item toits operating state [14]. Normally, corrective maintenance is an unplanned mainte-nance action that requires urgent attention that must be added, integrated with, orsubstituted for previously scheduled work. Corrective maintenance or repair is animportant element of overall maintenance activity.

    Preventive maintenance is an important element of a maintenance activity andwithin a maintenance department it normally accounts for a significant proportionof the overall maintenance activity. Preventive maintenance is the care and servicingby maintenance personnel to keep facilities in a satisfactory operational state byproviding for systematic inspection, detection, and correction of incipient failureseither before their development into major failures or before their occurrence [2,4].There are many objectives of performing preventive maintenance including improv-ing capital equipments productive life, reducing production losses caused by equip-ment failure, minimizing critical equipment breakdowns, and improving the healthand safety of maintenance personnel [5].

    This chapter presents various important aspects of both corrective maintenanceand preventive maintenance.

    12.2 TYPES OF CORRECTIVE MAINTENANCE

    Corrective maintenance may be grouped under the following five categories [2,4,6]:

    Fail repair:

    This is concerned with restoring the failed item or equipment toits operational state.

    Overhaul:

    This is concerned with repairing or restoring an item or equipmentto its complete serviceable state meeting requirements outlined in maintenanceserviceability standards, using the inspect and repair only as appropriatemethod.

    Salvage:

    This is concerned with the disposal of nonrepairable materials andutilization of salvaged materials from items that cannot be repaired in theoverhaul, repair, or rebuild programs.

    Servicing:

    This type of corrective maintenance may be required because ofa corrective maintenance action; for example, engine repair can result inrequirement for crankcase refill, welding on, and so on.

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    Maintainability, Maintenance, and Reliability for Engineers

    Rebuild:

    This is concerned with restoring an item or equipment to a standardas close as possible to its original state with respect to appearance, perfor-mance, and life expectancy. This is accomplished through actions such ascomplete disassembly, examination of all parts, replacement or repair ofunserviceable or worn components according to original specifications andmanufacturing tolerances, and reassembly and testing to original productionrequirements.

    12.3 CORRECTIVE MAINTENANCE STEPS, DOWNTIME COMPONENTS, AND TIME-REDUCTION STRATEGIES AT SYSTEM LEVEL

    Different authors and researchers have proposed different steps for carrying outcorrective maintenance [1,3]. For our purpose, we assume that corrective mainte-nance can be performed in the following five steps [4]:

    Failure recognition:

    Recognizing the existence of a failure

    Failure localization:

    Localizing the failure within the system to a specificpiece of equipment item

    Diagnosis within the equipment or item:

    Diagnosis within an item orequipment to identify specific failed part or component.

    Failed part replacement or repair:

    Replacing or repairing failed partsor components.

    Return system to service:

    Checking out and returning the system back toservice.

    Corrective maintenance downtime is made up of three major components asshown in Figure 12.1 [4,7].

    Active repair time is made up of six subcomponents: checkout time, preparationtime, fault correction time, fault location time, adjustment and calibration time, andspare item obtainment time [4,7].

    FIGURE 12.1

    Major corrective maintenance downtime components.

    Active repair time

    Administrative andlogistic time

    Components Delay time

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    145

    In order to improve corrective maintenance effectiveness, it is important toreduce corrective maintenance time. Some of the useful strategies for reducingsystem-level corrective maintenance time are [2,8]:

    Improve accessibility:

    Past experiences indicate that often a significantamount of time is spent accessing failed parts. Careful attention to acces-sibility during design can help to lower the accessibility time of parts and,consequently, the corrective maintenance time.

    Improve interchangeability:

    Effective functional and physical interchange-ability is an important factor in removing and replacing parts or components,thus lowering corrective maintenance time.

    Improve fault recognition, location, and isolation:

    Past experiencesindicate that within a corrective maintenance activity, fault recognition, loca-tion, and isolation consume the most time. Factors that help to reducecorrective maintenance time are good maintenance procedures, well-trainedmaintenance personnel, well-designed fault indicators, and unambiguousfault isolation capability.

    Consider human factors:

    During design, paying careful attention to humanfactors such as selection and placement of indicators and dials; size, shape,and weight of components; readability of instructions; information processingaids; and size and placement of access and gates can help lower correctivemaintenance time significantly.

    Employ redundancy:

    This is concerned with designing in redundant partsor components that can be switched in during the repair of faulty parts sothat the equipment or system continues to operate. In this case, althoughthe overall maintenance workload may not be reduced, the downtime ofthe equipment could be impacted significantly.

    12.4 CORRECTIVE MAINTENANCE MEASURES

    There are many corrective maintenance-related measures. Two of those measuresare presented below [4,8,9].

    12.4.1 M

    EAN

    C

    ORRECTIVE

    M

    AINTENANCE

    T

    IME

    This is an important measure of corrective maintenance and is defined by

    (12.1)

    where

    CMMT

    is the mean corrective maintenance time, is the failure rate of the

    i

    th equipment element, and

    CMT

    i

    is the corrective maintenance time of the

    i

    thequipment element.

    CMMTCMT

    i i

    i

    =

    i

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    Maintainability, Maintenance, and Reliability for Engineers

    Usually corrective maintenance times are described by normal, lognormal, andexponential probability distributions. Examples of the types of equipment that followthese distributions are:

    Normal distribution:

    Corrective maintenance times of mechanical or elec-tromechanical equipment with a remove and replacement maintenance con-cept often follow this distribution.

    Lognormal distribution:

    Corrective maintenance times of electronic equip-ment that does not possess built-in test capability usually follow this dis-tribution.

    Exponential distribution:

    Corrective maintenance times of electronicequipment with a good built-in test capability and rapid remove andreplace maintenance concept often follow this distribution.

    12.4.2 M

    EDIAN

    A

    CTIVE

    C

    ORRECTIVE

    M

    AINTENANCE

    T

    IME

    This is another important measure of corrective maintenance. It usually provides thebest average location of the sample data and is the 50th percentile of all values ofcorrective maintenance time. Median active corrective maintenance time is ameasure of the time within which 50% of all corrective maintenance activities canbe performed. The computation of this measure is subject to the probability distri-bution describing corrective maintenance times. Thus, the median of correctivemaintenance times following a lognormal distribution is expressed by [2,8]

    (12.2)

    where

    MACMT

    is the median active corrective maintenance time.

    12.5 MATHEMATICAL MODELS FOR PERFORMING CORRECTIVE MAINTENANCE

    Many mathematical models are available in the published literature that can be usedin performing corrective maintenance. This section presents two such models. Thesemodels take into consideration item failure and corrective maintenance rates andcan be used to predict item, equipment, and system availability, reliability, probabilityof being in a failed state (i.e., undergoing repair or corrective maintenance), meantime to failure, and so on.

    12.5.1 M

    ATHEMATICAL

    M

    ODEL

    1

    This model represents a system that can be in either operating or failed state. Thefailed system is repaired back to its operating state. Most industrial systems,equipment, and items follow this pattern. The system-state space diagram is shown

    MACMT antiCMT

    i i

    i

    =

    loglog

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    147

    in Figure 12.2 [9]. The numerals in the box and circle in Figure 12.2 denote systemstates. The following assumptions are associated with the model:

    All system failures are statistically independent. System failure and repai

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